Notes

Abstract:

Prussian blue analogues (PBA) are a class of coordination polymers some of which are known to exhibit stimuli switchable magnetism. The mechanism in which these PBA undergo a change in magnetization is through a Charge Transfer Induced Spin Transition (CTIST). Of the materials that exhibit a CTIST, the PBA cobalt hexacyanoferrate (CoFe) is the most extensively studied example. Recently PBA heterostructures, pairing two different components in a single material, have emerged, herein CoFE and nickel hexacyanochromate (NiCr) are paired. Heterostructures exhibit changes in structural properties compared to pure materials. We propose that the interface coupling between the lattices induces strain, which leads to new structural behaviors. To better understand these behaviors we investigate a series of morphologies. NiCr by itself does not undergo a CTIST but when coupled to a CoFe core the NiCr lattice parameter contraction is greater than that of a thermal contraction. The extent of this contraction minimized when the shell thickness is increased. The effect of strain is investigated by varying shell thicknesses. A second series, where CoFe is the shell material encasing a NiCr core, is investigated. For these inverse particles the shell undergoes a CTIST, the NiCr core does not exhibit strain. These are then compared to a third series, CoFe hollows; which appear to be spin trapped until the shell reaches 70 nm in thickness.

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1 MORPHOLOG ICAL EFFECTS ON THE PHYSICAL PROPERT IES OF Rb j Co k [Fe(CN) 6 ] l n H 2 O PRUSSIAN BLUE ANALOG UE S By KATHERINE ALICE SOMODI A THESIS PRESENTED TO THE UNDER GRADUATE CHEMISTRY DEPARTMENT OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFI LLMENT OF THE REQUIREMENTS FOR UPPER DIVISION HONORS UNIVERSITY OF FLORIDA 2013

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2 2013 Katherine Alice Somodi

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3 To Opus Coffee

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4 ACKNOWLEDGMENTS I would like to thank Matthew Andrus for inviting me into the group as a freshman, and Dr. Daniel Talham for giving me the chance to learn and work throughout my undergraduate experience. I appreciate the introduction to lab work that I received by Matthieu Dumont and Emily Pollard who helped hone my lab skills. I especially appreciate Matthew Andrus for working closely with me for the majority of my experience, and being a constant source for advice, encou ragement, and knowledge through research, classes, and application processes Thanks to Olivia Risset Caue Ferrera and UF Major Analytical Instrumentation Center for TEM images and EDS data. I extremely appreciate the synthesis and materials help I garner ed from Carissa Li and Olivia Risset. The Talham group members were a consistently helpful think tank, Corey Gross, Hao Liu, Divya Rajan, Akhil Ahir, Yichen Li, Allison Garnsey, Ashley Felts, and Carolyn Averback. I would like to thank Dr. Khali l Abboud for XRD time and data in conjunction with Matthew Andrus. Finally, my best friends who have supported and uplifted me: Kayla, Jessica, Rebecca, and my boyfriend Nicolas. My family has been a source of encouragement throughout life and this process, Gail, Daniel, and Daniel C. Somodi. The communities I have been a part of have played a role in who I am, my motivation, drive, and stress relief, for that the Gator Wesley Foundation and Warren Willis Unit ed Methodist Camp deserve recognition Importantly, I p raise God for this intricate world, science and determination. This work was supported, in part, by the NSF Grant No. DMR 105581 (DRT).

9 Abstract of Thesis Presented to the Department of Chemistry of the University of Florida MORPHOLOG ICAL EFFECT S ON PHYSICAL PROPERT ES IN Rb j Co k [Fe(CN) 6 ] l n H 2 O PRUSSIAN BLUE ANALOGUES By Katherine Alice Somodi December 2013 Chair: Daniel Talham Major : Biochemistry Prussian blue analogues (PBA) are a class of coordination polymers some of which are known to exhibit stimuli switchable magnetism. The mechanism in which these PBA undergo a change in magnetization is through a Charge Transfer Induced Spin Transition (CTI ST). Of the materials that exhibit a CTIST, the PBA Cobalt hexacyanoferrate (CoFe) is the most extensively studied example. Recently PBA heterostructures, pairing two different components in a single material, have emerged, herein CoFe and Nickel hexacyano chromate (NiCr) are paired. These heterostructures exhibit changes in structural properties compared to pure CoFe and NiCr materials. We propose that the interface coupling between the two lattices induces strain, which leads to new structural behaviors. T o better understand these behaviors we investigate a series of morphologies. At room temperature there does not appear to be significant strain at the interface due to the lattice mismatch. However when cooled through the thermal CTIST both the NiCr and Co Fe lattices are strained. NiCr by itself does not undergo a CTIST but when coupled to a CoFe core the NiCr lattice parameter contraction is greater than that of a thermal contraction. The extent of this contraction minimized when the shell thickness is in creased. Therefore in this study the effect of

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10 strain is investigated by varying shell thickness. As the NiCr shell grows from 10 nm to 95 nm, the CTIST of CoFe is diminished and the NiCr bulk character dominates. The thinnest NiCr shell, 10 nm, exhibits t he most strain, induced by the CoFe core, which has the dominating core characteristics. A decreasing lattice constant for CoFe illustrates this as the NiCr shell increases. A second series where CoFe is the shell material encasing a NiCr core (inverse to the first series) were investigated. The CoFe shell can only undergo a complete CTIST when the shell thickness is 40 nm or greater. Thinner shells result in partially inhibited transitions. Even though the shell is undergoing a CTIST, the NiCr core does not exhibit strain. A third series of hollow CoFe particles are prepared to enable study of the air surface interface and its influence on the CTIST. Hollow CoFe shells do not undergo a complete CTIST until the shell s are 70 nm thick. The hollow CoFe particles that undergo a CTIST are about twice the thickness required to completely transition for inverse heterostructures. This suggests that surface and morphology of the particles influence the CTIST.

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11 CHAPTER 1 INTRODUCTION Prussian Blue Analogues The past decades have seen an upswing in Prussian Blue Analogue (PBA) research, especially with respect to molecular magnetism which has been observed in several systems including ACo[Fe(CN) 6 ], AMn[Fe(CN) 6 ], ACo[Os(CN) 6 ] The cobalt hexacyanoferrate, ACo[Fe(CN) 6 ]nH 2 O (CoFe) was the first system reported to exhibit a t hermal and light induced change in magnetization ha ve been meticulously studied. External stimuli have an effect on the electronic properties, meaning tha t switching temperature, light, magnetism, change the electronic and structural make up. This change from high spin (HS) to low spin (LS), for example, is accompanied by a physical change or unit cell contraction. The change in magnetization results from a charge transfer induced spin transition (CTIST) from C o (II) N C Fe (III) to Co (III) N C Fe (II) The change is a result of the cobalt changing from high spin to low spin. (1 4 ) The ability to manipulate the electronic properties and the material itself lea ds to p ossible applicat ions in medicine 22 25 and technology such as biosensors and batteries 26 28 PBA are in dexed to a face centered cubic unit cell formation with mixed transition metals brid ged by cyanide. Typically the molecular formula is expressed by the following, A j M k [M (CN) 6 ]l n H 2 O, where A is an alkali metal, M is a divalent metal, and M is a trivalent metal To achieve charge balance, either alkali metals intercalate the cubic structure, or water fills cyanometallate vacancies in the lattice Figure 1 1. These limiting examples produce HS and LS trapped material in CoFe In most ci r cumstances both alkali metals and water are part of the lattice, which results in a substoichiometric molecular formula. Studies have been done with the li miting molecular formulas and at

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12 intermediate stoichoimetries It has been noted that the li g a n d field strength surrounding the Co ion is determined by the number of coordinated wanters 6 When water completes the lattice coordination sites it stab i lizes the HS state due to a weaker ligand field strength. T he cyanide bridge, which mediates the electron transfer for intermediate stoichometries, assists in the CTIST between the high and low spin states making the CoFe PBA a prime candidate for investigating the stru ctural and magnetic properties This PBA was first studied for its photoinduced magnetism. 1, 5 Cobalt Hexacyanoferrate Prussian Blue Analogues In the case of CoFe PBA the photomagnetism is a result of the CTIST, from a high temperatur e phas e and low temperature phase T he HT phase is mostly Co 2+ HS sites while LT phase consists of Co 3+ LS sites When thermal energy is added or removed from the system the HS or LS states are stabilized, which also results in a structural change 7 The Co N bond contracts with the decrease in temperature, as indicated by XRD data, with lattice constants (a) of 10.30 (HT) to 9.96 (LT). 8,9,10 Multiple factors may tune the potential energy between the states, HS and LS Altering the ligand field s trength around the Co by experimenting with stoichiometry 6 and the vacancies in the lattice has been documented, as well as changing the alkali metal. 29 Temperature and light have also been notable stimuli for inducing the CTIST, therefore changing potenti al energy. 11 15 Above 200 K, the HT phase is typically favor ed however when below 200 K the LT phase stabilized Light can used to excite the material from LT to HT phase below 150 K and below 25 the HT phase is know to order ferrimagneti callly 9,11 The t hermal hysteresis that occurs in CoFe PBA is illustrated

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13 in Figure 1 2 as a function of magnetic susceptibility. 6, 12 Due to the phase changes that are accompanied by electronic and structural changes, the CTIST has been studied in differing terms. 6 ,13 Heterostructures and hollow particles have become important for probing the effects that not only light, temperature, and pressure have on the structure; but also the effect of morphology of the material and how the surface interface plays into phase cha nges. Heterostructured Prussian Blue Analogues Heterostructures are multicomponent materials, where in this case the core is a PBA particle, which provides a nucleation site for the growth of a PBA she ll surround ing th e core. These core shell heterostruc tures are defined as c ore@shell. When PBA with different properties are combined into a single material, there abounds new ways for controlling structural and electronic properties. 14,15 Herein, two different materials, one that exhibits a CTIST and is pho toactive while the second material does not have either property are studied in combination providing different electronic and physical, structural properties. In addition to CoFe, t he other material in these studies is N ick el hexacyanochromate (NiCr). Herein the size of the NiCr shell on the CoFe core is varied in thickness to probe the effect in the core@shell morphology. Then the thickness of a CoFe shell on a NiCr core is studied. Of particular interest are the chemical and physical property changes when coupled materials are induced to phase changes which can be studied because the bulk properties of each component are known When NiCr and CoFe are heterostructured, there is a photoinduced decrease in magnetization at LT, when the shell is large eno ugh, and an increase in ordering temperature for the photoexcited magnetism. 14,15,16 The observation that the CoFe@NiCr material stays

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14 photomagnetic at higher temperature, leads to a major research interest in these materials. The morphology effects of the CoFe@NiCr, NiCr@CoFe, and hollow CoFe are compared herein the interface study is attempted but information is lacking as to the role of the surface on the particles Hollow Cobalt Hexacyanoferrate Prussian Blue Analogues Recently hollow PBA particles have been reported 17 Prussian blue analogue hollows are successfully synthesized using a sacrificial template, in this case another PBA, Ma nganese hexacyanoferrate (MnFe). 17 20 The PBA of interest forms a core@shell before becoming a hollow shell. It has been reported that hollow CoFe particles do not undergo a CTIST The current rationale for these particles remaining in the HT phase is either a morphology or surface effect. It is our aim to investigate how the physical properties of these hollow CoFe particles change as the shell thickness increases. The hollows are essentially surface, which enables a study on surface effects and characterization of these primarily surface particles. The thickness of the hollow shell determines their ability to undergo a CTIST At 10 nm, the thinnest shell, t he particles are LS trapped ; when grown larger up to 70 nm, the shell begins to exhibit a fraction of the character of a core particle, in the ability to undergo a CTIST. Studying the thicknesses of these hollow shells can be taken as studying simply the surface of the PBA, in this case CoFe. When the heterostructured materials are studied and characterized, it is seen that the surface penetrates into the core, and that the core properties penetrate the shell. The hollows have no other material in coordination when washed properly, therefore, are able to

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15 attribute strain or changes to the morphology of the particle. Herein the hollow s are compared to heterostructures such th at surface effects, size, and overall morphology can be characterized. The unique properties of the particles are attributed to their morphologies. The particles are synthesized for careful cross comparison such that morphologies can be utilized to discov er reasoning behind physical changes as a result of the CTIST. Heterostructures: CoFe@NiCr inverse heterostructures : NiCr@ CoFe, and Hollow CoFe can correlate the effects lattice coupling and morphology have on the physical properties of CoFe PBAs. Figure 1 1. The limiting PBA unit cells. The left illustrates the alkali metal cationic charge balance, the right, when water coordinate s to preserve charge balance

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16 Figure 1 2. Magnetic susceptibility times temperature, versus t emperature plots. Sodium the alkali metal in this series of PBA, is varied and the CTIST is tuned. Reprinted with permission from Shimamoto, et al. copyright 2002 American Chemical Society. 6

22 Characterization Fourier Transform Infrared Spectroscopy (IR) measurements were performed on a Thermo Scientific Nicolet 6700 spectrometer depos i ting <1 mg of sample on a KBr pellet. The measurements took place in the 1800 2500 cm 1 range with a 1 cm 1 resolution over 16 scans. Both Energy Dispersive X Ray Spectroscopy ( EDS ) and Transition Electron Microscope ( TEM ) were conducted on the JEOL 2010F Field Transmission TEM at the Major Analytical Instrumentation Center at the University of Florida. Using water and acetone suspensions CoFe PBA samples were deposited onto a 400 mesh copper grid with ultrathin carbon film on a holey carbon support film, f rom Ted Pella, Inc. Particle size analysis was determined by measuring more than 150 31 Combustion analysis was performed at the University of Florida Spectroscopic Services Laboratory to determine carbon, hydrogen, and nitrogen (CHN) content. Powder X ray diffraction (XRD) measurements were performed on a Bruker DUO diffractomer using Cu K radiation from an I S source, multi layered mirror optics and an APEXII CCD area detector (1024 x 1024) at 150 mm from the sample. Samples were packed into capillary tubes, 0.3 mm in diameter, boron rich and thin walled, from Charles Supper Company. Between 5 and 89 diffraction patterns were collected, with a 600 s/image collection time. The temperature, cooling, was controlled by nitrogen gas flow, regulated by an Oxford Cryostream. APEXII generated the raw data and this was analyzed using Fullprof Rietveld refinem ent software 21

24 CHAPTER 3 RESULTS AND DISCUSSION Analysis of Heterostructure PBA Three different PBA heter o structu r es were studies, CoFe@NiCr, NiCr@CoFe, and hollow CoFe For a schematic on morphology see Figure 3 1. To determine the M 2+ /M 3+ ratio, EDS was conducted, as it identifies the elements and their relative abundance The molecular formu las were determined by EDS and CHN ; where the alkali metal content was determined by charge balance. FTIR was used to confirm the components of the he terostructures. Morphology and particle size was evaluated by TEM. Table 3 1, data The TEM data can also be seen in Figures 3 2,4. Powder X Ray Diffraction (XRD) The powder XRD measurements were important for determining the structure of the PBA coordination polymers. The unit cells for NiCr and CoFe were determined using at 300 K and 100 K Furthermore, b roadening of the re f lections can provide insight towards latt ice strain. There are a couple of possibilities attributed to peak broadening such as non stoichiometric ratios in the compound or lattice mistakes Here anisotropic strain is the primary component of peak broadening which is observed in the cooled, LT ph ase Movement of the highest intensity peaks indicate contraction of the unit cell, which in CoFe, bare particles, is very common and results from the CTIST slight changes in lattice constant for NiCr is also observed (Table3 1) The unit cells for NiCr and CoFe were indexed to a FCC lattice, using the 200, 220, 400, 420, reflections The peak center was determined using a pseudo Voigt fit. The reflections determine the unit cell length, which when compared between the high

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25 and low temperatures can be used to indicate physical state changes Figure 3 5 is a compilation of the diffraction pattersn for each sample in the HT and LT phases, A to B respectively The full width at half maximum (FWHM) from the Viogt fit of the most intense reflections helps to determine broadening during cooling a characteristic of strain A change in the width of the peak can be compared on stacked plots and quantified; the causes of this are to follow. Pure Bare PBA The bare terminology implies that the material is not modified by heterostructure, or otherwise different morphology. The particles are in the FCC lattice. Their molecular formula, as listed in Chapter 2, is near identical to the other particles utilized for the study, thus is an ideal ref erence The particles used for this study were in the HT phase at 300 K When cooled to 100 K the particles undergo a CTIST and the unit cell contracts to the LT phase. The unit cell contraction has a lattice constant, a =0.35 Pure CoFe sees broadening in the hk0 ( i.e. 420) reflection s at low temperature NiCr does not undergo a CTIST, the XRD information illustrates this lack of phase change, see Table 3 1 The XRD data demonstrates th is lack of phase change, though there is a subtle shift in the reflec tions which is attributed to thermal contraction The bare materials are a reference f or the heterostructure s and hollow particles to be compared. The aspects such as particle size were carefully constructed by synthetic techniques such as addition rate and stir time such that the sizes would be comparable and logical.

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26 C o F e@ N i C r Heterostructured PBA The heterostructures were similarly studied. Wh en cooled from 300 K to 100 K the CTIST appears to be inhibited to greater extents with increasing shell thickness. The 15 nm NiCr shell shows signs of strain, see Figure 3 5, sample 2 2, that are not typically present for NiCr The lattice constant for th e 15 nm NiCr shell sample decreases more than any other heterostructure or bare NiCr materials In the 40 nm shell materials, there is less of a decrease in lattice constant for the NiCr shell as seen in Table 3 1. Along with this is a a decreased lattice constant, meaning less of a contraction, for the CoFe core. The largest NiCr shell, has essentially overcome the strain imposed by the CoFe core, referencing Table 3 1 sample 2 5. The CoFe CTIST has been depleted as evidenced by the lat tice constant. For CoFe@NiCr core@shells the size of the NiCr shell affected the extent to which the CTIST was able to occur in the CoFe. The CoFe/NiCr interface infl u ences the extent to which both lattices contract when cooled to 100 K. If the shell thick ness is small, the strain is quite apparent and the CTIST in CoFe is closer to the pure material CITST The change in unit cell, a decrease, illustrates the CTIST associate d physical change. Furthermore, for the thinnest N iCr shell at low temperature has a much smaller lattice parameter than the pure NiCr material. This suggests the change in CoFe lattice is influencing the shell material. Additionally, t he hk0 reflections of NiCr shell are broaden ed ; another indication of strain at the interface. This is measured by an increase in FWHM. The CoFe must influence the shell for the first several layers. It is asserted that a rigid NiCr shell inhibits the CTIST due to lattice coupling at the interface as the NiCr increases in thickness. The shell of 95 nm approaching the size of the core particle, diminishes the CTIST. Though, it is not until

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27 about 60 nm that the shell behaves like the p ure NiCr material. Strain on NiCr decreases as the thickness of the shell increases. NiCr@CoFe Heterostructured PBA The NiCr@CoFe core@shell particles are an inverse system to the previous heterostructure samples and give the opportunity to further probe morphology effects. For this series CoFe is the shell material with thicknesses ranging from 10 to 40 nm. The thinnest CoFe shell (10 nm) does not undergo a CTIST Two factors could be limiting this transitions, either the NiCr core inhibits the transition or the shell is so thin it acts purely as a surface and cannot undergo the CTIST This study ai ms to better understand this inhibition. The ratio of the Co/Fe suggests that this shell should undergo a CTIST, however, the lattice coupling or surface effects prevent the phase change from happening. By increasing the CoFe shell thickness the material b egins to undergo a CTIST See Table 3 1, samples 2 7,8,9. A s the shell thickness is increase d to 40 nm a complete transition occurs. This suggests the morphology does not entirely inhibit the CTIST. However, it is unclear if the surface or the mor phology inhibits the CTIST in the thinnest shell. This point will be discussed in more detail later on in this report. It is also important to note that t he NiCr does not show evidence of strain, by comparison of FWHM during cooling of the inverse heterostructures T here is no significant change in the NiCr lattice parameter upon cooling to 100 K as observed in the CoFe@NiCr heterostructures. However, the CoFe shows evidence of strain possibly from the interface with NiCr and the particle surface. The complexity of this system makes it difficult t o differentiate morphology and surface effects. Therefore, h ollow CoFe particles were investigated to provide further insight into th e source of strain on the CoFe shell.

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28 Hollow CoFe PBA The hollow CoFe PBA shells studied allow for possible determination of the source of strain in the heterostructure particles as hollows serve as a prime example of possible surface strain Four sets of h ollow particles were grown wi th shell thickness ranging from 10 to 70 nm. The 10 nm CoF e hollow particles did not undergo a CTIST as previously reported in the literature. When increased in thickness, a lattice contraction occurs. However, the contraction is incomplete for hollow CoFe particles with shell thicknesses less than 70 nm. See Fig ure 3 5 ,7 for the hollow particle X Ray diffraction patter n s. Hollow particles require thicker shells than the NiCr@CoFe shells to undergo a complete CTIST. This strain could be attributed to the morphology of the hollow shell, simply being a shell, or having surface effects. From this study, the difference of strain origin is unclear. However analyzing the comparison morph ologies could be enlightening. The NiCr@CoFe particles undergo a complete CTIST when the shell thickness is 40 nm Meanwhile, hollow CoFe particles need a thickness of 70 nm to undergo a complete CTIST Despite having the same morphology the hollow partic les need to be nearly twice as thick. This suggests that the hollow cubes are not flexible enough to go through the CTIST which is an attribute of surface effects. Thereby increasing the shell thickness reduces the overall surface nature of the hollow par ticles and subsequently bulk properties begin to dominate. The fact that a CTIST is possible in a hollow morphology indicates that it is not the morphology itself preventing a CTIST in lower thicknesses. Compare the 2 13 sample to the smaller shell hollow particles ray diffraction patterns, Figure 3 7 We assert that the surface, penetrating the few available

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29 layers of the thin hollow shells, inhibit the CTIST. The surface, compared to larger thicknesses, has less ability to penetrate the shell and exhibit any control on the phase change or cell contraction. Several param e ters still influence the CTIST. It is difficult to conclude completely and differentiate each effect H owever with these samples we can conclude different factors mediate the CTIST in CoFe PBA particles, heterostructures, and nanocubue s th r ough differences in s urface, morphology and strain effects. This offers new insights into ways to tune these material properties. Figure 3 1. A schematic illustrating the morphology of the part icles included in the study.

33 CHAPTER 4 CONCLUSIONS The size, shape, and formation of the PBAs studied all have an effect on the physical properties of the material itself. Collectivel y, these morphology effects play into unit cell changes, phase behavior and photomagnetism. The strain on CoFe core@shell PBAs when undergoing a CTIST is more intense when CoFe is the core as opposed to when NiCr is the core. Hollows behave slightly differ ently from bulk CoFe or CoFe shell materials, in that they are spin trap ped until thick enough to escape influence by a surface interface. The character of hollow particles is therefore not simply a question of their morphology, but of their surface effect s. It has been found that at the interface of materials, especially in the core@shells, the properties are different and the strain on the material may be caused by this character difference. When the thickness of the shells or hollows grow, spin trapped states or strain decrease, as the original material character has a chance to predominate. In the future, magnetic measurements must be done, such that the magnetic properties of the hollows, especially, may be compared to the known magnetic behavior of b ulk, nano, and heterostructured materials. The XRD was able to study the unit cell changes or strain as the temperature induced HS to LS changes in materials. Alternatively important, would be to see the changes in magnetization as hollows undergo the CTIST, as the thinnest would be spin trapped and the hollow particles above 70 nm may exhibit magnetizati on changes relatively similar to those observed in pure bare particles. These measurements may be integral in determining the use of the surface area on hollows and ways to design materials according to phase or magnetic need or use.

37 BIOGRAPHICAL SKETCH Katherine Somodi was born in Sarasota, Florida, and after a brief stint in North Carolina in elementary school, was raised in Bradenton, Florida. Sh e attended Lakewood Ranch High S chool where she was class Vice President and salutatorian. She will be graduating with a Bachelor of Science in Biochemistry from the U niversity of Florida Upon graduation she will continue volunteering at Helping Hands Clinic, as an eligibility specialist in an administration role facilitating the provision of health care to homeless, uninsured, and impoverished in Gainesville. She wil l also continue to work as a barista at Opus Caf. She will be regaining a role in The Vagina Monologues, Gainesville 2014, benefitting Peaceful Paths In her free time, Katherine enjoys rock climbing, heights, and outdoor adventures; such as zip lining o n a volcano in Costa Rica. She has enjoyed the view of Machu Picchu from Wayna Picchu in Peru and th e beauty of Lake Balaton in Hungary. In the year to come she will be matriculating to medical school in pursuit of her life long dream of being a doctor.